Hypersonic aircraft, which may include manned and unmanned spacecraft and reentry vehicles, must have outer skins that can withstand high temperatures caused by atmospheric friction, commonly experienced upon atmospheric reentry. In addition, the skins of such vehicles must withstand the very low temperatures encountered in the cold of space, such as might be encountered while in orbit. Aluminium alloys, which typically are used for the outer skins of such vehicles, cannot withstand the high temperatures (up to 3,000° F./1,650° C.) that may be encountered upon atmospheric reentry. In order to protect the outer skin of such aircraft, thermal protection systems (TPS) have been developed.
Such thermal protection systems typically include an array of tiles that possess resistance to high temperatures of the surrounding atmosphere and also insulate the underlying aluminum alloy skin of the aircraft. The individual tiles typically are square or rectangular in shape and made from silica derived from very pure quartz sand or a combination of silica and aluminoborosilicate or alumina. The tiles are adhesively attached to the skin of the vehicle on which they are mounted, and in most applications are affixed to Nomex felt strain isolation pads (SIPs) with a room temperature vulcanizing (RTV) silicone adhesive that in turn is bonded to the skin of the vehicle. This can also apply with other types of insulative units than ceramic tile, such as high-temperature composite components (ceramic matrix composite material such as aluminosilicate/alumina, alumina/mullite, carbon/silicon carbide, silicon carbide/silicon carbide) or high-temperature metallic tiles (Inconel or titanium tiles filled with insulation).
Ideally, the thermal protection system tiles would be closely spaced on the outer skin of a vehicle, with very little or no gap between adjacent tiles, or with the gaps between tiles filled tightly. However, such filling and/or spacing can lead to high lateral stresses on installed rigid tiles, particularly during cold soak that occurs when the vehicle is in orbit and the skin and other underlying metallic structure is at its greatest contraction. It is necessary to mount the tiles on the vehicle with sufficient spacing between the tiles to accommodate cold soak conditions encountered when the vehicle is in orbit. However, during vehicle reentry, the temperatures encountered by the vehicle cause the underlying metallic skin to expand, thereby increasing the gaps between adjacent tiles. This exposes the underlying metallic skin to the high temperatures encountered in reentry. Accordingly, there is a need for a thermal seal that bridges the gaps between adjacent tiles of a thermal protection system, but does not impose unacceptably high lateral stresses on the tiles when the gaps between tiles contract during cold soak conditions.